A Water-Free In Situ HF Treatment for Ultrabright InP Quantum Dots

Indium phosphide quantum dots are the main alternative for toxic and restricted Cd-based quantum dots for lighting and display applications, but in the absence of protecting ZnSe and/or ZnS shells, InP quantum dots suffer from low photoluminescence quantum yields. Traditionally, HF treatments have been used to improve the quantum yield of InP to ∼50%, but these treatments are dangerous and not well understood. Here, we develop a postsynthetic treatment that forms HF in situ from benzoyl fluoride, which can be used to strongly increase the quantum yield of InP core-only quantum dots. This treatment is water-free and can be performed safely. Simultaneous addition of the z-type ligand ZnCl2 increases the photoluminescence quantum yield up to 85%. Structural analysis via XPS as well as solid state and solution NMR measurements shows that the in situ generated HF leads to a surface passivation by indium fluoride z-type ligands and removes polyphosphates, but not PO3 and PO4 species from the InP surface. With DFT calculations it is shown that InP QDs can be trap-free even when PO3 and PO4 species are present on the surface. These results show that both polyphosphate removal and z-type passivation are necessary to obtain high quantum yields in InP core-only quantum dots. They further show that core-only InP QDs can achieve photoluminescence quantum yields rivalling those of InP/ZnSe/ZnS core/shell/shell QDs and the best core-only II–VI QDs.

3 effects were taken into account by using effective core potentials. Geometry optimizations were performed at 0 K and in the gas phase.
To confirm the orbital localization of the trap state, the inverse participation ratio (IPR) 6,7 of an electronic state is used. We define IPR as: Where P a,i is the weight of a molecular orbital i on a given atom a expanded in an atomic orbital basis. The IPR can be used to estimate the number of atoms that contribute to an electronic state.
The value can range from , meaning the contribution is equally distributed 1 # atoms present in the system over all atoms, to 1, meaning that the contribution comes from 1 atom.
Crystal-orbital overlap population (COOP) is used to give insight on the bonding or anti-bonding nature of an electronic state. [8][9][10] Positive COOP values correspond to a bonding interaction, whereas negative COOP values correspond to an anti-bonding interaction. A value close to 0 is a non-bonding interaction.
IPR & COOP calculations were carried out using the QMFlows Python package. 11 Figure S1. Solution 1H spectra of the compounds used in the in-situ HF treatment, before and after their reaction. Spectra were referenced according to the residual solvent peaks of toluene-d8 (2.08 ppm). As can be observed from the shift in peak 11 (no residual peak at 2.5 ppm after the reaction), full conversion of the amine is achieved when equimolar amounts are reacted. 5 Figure S2. Solution 31 P NMR spectra of aliquots taken from treatments at different temperatures.
The PH 3 peak can clearly be observed at -239 ppm. Free TOP is also present in all spectra at -29 ppm. At -210 ppm, a final peak can be observed, but only in the RT and 90 °C spectra. This peak shows at the typical shift of InP (see solid state NMR spectra in Figure S10), and is thus ascribed to small InP clusters/complexes in solution.       S10. X-ray diffraction patterns of InP QDs before and after the in-situ HF treatment. After the treatment the peak at 20°, ascribed to ordered palmitate ligands, is decreased in intensity. Figure S11. 31 P NMR shifts of various small InP structures as calculated using DFT. Shifts were referenced using the calculated shift of H 3 PO 4 . For PO x species, lower chemical shift is observed for increasing x, which mirrors experimental observations. P bound to four InF 3 was calculated to have a lower chemical shift than P bound to four In(carbonate) 3 . This is in accordance with our observations after the HF treatment, were the 31 P chemical shift becomes more negative after In(PA) 3 has been replaced with InF 3 on the InP surface.    Adding triethylamine before starting the treatment results in less HF etching, while achieving the same increase in PLQY. Less InP is converted to InF 3 as can be seen by the reduced absorption 20 drop compared to the HF treatment. There is also no absorption or PL blueshift observed after the treatment. Figure S17. O1s and C1s XPS spectra of the InP QDs before and after the in-situ HF treatment. It should be noted that after the in-situ HF treatment, less carbonate ligands are present on the InP surface than before the treatment. The dropcasting procedure of the samples after the treatment also resulted in thinner QD layers with a lower coverage of the substrate. Because of these factors, 21 the signal from contaminations (organic compounds on the substrate) in the O1s and C1s spectra is expected to be higher in the in-situ HF treated samples relative to the untreated samples, which makes a comparison of these spectra difficult. Figure S18. 13 C ssNMR spectra. After the in-situ HF treatment, octylamine is bound to the surface in addition to some remaining palmitates.